x86/guest: setup event channel upcall vector

x86/guest: setup event channel upcall vector

And a dummy event channel upcall handler.

Note that with the current code the underlying Xen (L0) must support
HVMOP_set_evtchn_upcall_vector or else event channel setup is going to
fail. This limitation can be lifted by implementing more event channel
interrupt injection methods as a backup.

Register callback_irq to trick toolstack to think the domain is
enlightened.

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>